Open Research OnlineThe Open University’s repository of research publicationsand other research outputs

Extra-terrestrial construction processes -advancements, opportunities and challengesJournal ItemHow to cite:

Lim, Sungwoo; Levin Prabhu, Vibha; Anand, Mahesh and Taylor, Lawrence (2017). Extra-terrestrial constructionprocesses - advancements, opportunities and challenges. Advances in Space Research, 60(7) pp. 1413–1429.

For guidance on citations see FAQs.

c© 2017 COSPAR. Published by Elsevier Ltd. All rights reserved.

https://creativecommons.org/licenses/by-nc-nd/4.0/

Version: Accepted Manuscript

Link(s) to article on publisher’s website:http://dx.doi.org/doi:10.1016/j.asr.2017.06.038

Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.

oro.open.ac.uk

Accepted Manuscript

Extra-terrestrial construction processes - advancements, opportunities and chal-lenges

Sungwoo Lim, Vibha Levin Prabhu, Mahesh Anand, Lawrence A. Taylor

PII: S0273-1177(17)30464-7DOI: http://dx.doi.org/10.1016/j.asr.2017.06.038Reference: JASR 13292

To appear in: Advances in Space Research

Received Date: 6 March 2017Revised Date: 15 June 2017Accepted Date: 20 June 2017

Please cite this article as: Lim, S., Prabhu, V.L., Anand, M., Taylor, L.A., Extra-terrestrial construction processes- advancements, opportunities and challenges, Advances in Space Research (2017), doi: http://dx.doi.org/10.1016/j.asr.2017.06.038

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customerswe are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting proof before it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Extra-terrestrial construction processes - advancements, opportunities and challenges

Dr Sungwoo Lim Corresponding author: sungwoo.lim@open.ac.uk

Research Fellow in Space, School of Physical Sciences, Faculty of Science, Technology, Engineering and Mathematics, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK Ms Vibha Levin Prabhu PhD student, School of Engineering and Innovation, Faculty of Science, Technology, Engineering and Mathematics, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK Dr Mahesh Anand Reader of Planetary Science and Exploration, School of Physical Sciences, Faculty of Science, Technology, Engineering and Mathematics, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK Dr Lawrence A. Taylor Distinguished Professor of Earth and Planetary Sciences, Director of the Planetary Geosciences Institute, The University of Tennessee, Knoxville, TN 37996, USA

2

Extra-terrestrial construction processes - advancements, opportunities and challenges

Sungwoo Lim1†, Vibha Levin Prabhu2, Mahesh Anand1, Lawrence A. Taylor3

† Corresponding author: sungwoo.lim@open.ac.uk 1 School of Physical Sciences, Faculty of Science, Technology, Engineering and Mathematics,

The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 2 School of Engineering and Innovation, Faculty of Science, Technology, Engineering and Mathematics,

The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 3 Planetary Geosciences Institute, University of Tennessee, Knoxville, TN 37996, USA

Abstract Government space agencies, including NASA and ESA, are conducting preliminary studies on building alternative space-habitat systems for deep-space exploration. Such studies include development of advanced technologies for planetary surface exploration, including an in-depth understanding of the use of local resources. Currently, NASA plans to land humans on Mars in the 2030s. Similarly, other space agencies from Europe (ESA), Canada (CSA), Russia (Roscosmos), India (ISRO), Japan (JAXA) and China (CNSA) have already initiated or announced their plans for launching a series of lunar missions over the next decade, ranging from orbiters, landers and rovers for extended stays on the lunar surface. As the Space Odyssey is one of humanity’s oldest dreams, there has been a series of research works for establishing temporary or permanent settlement on other planetary bodies, including the Moon and Mars. This paper reviews current projects developing extra-terrestrial construction, broadly categorised as: (i) ISRU-based construction materials; (ii) fabrication methods; and (iii) construction processes. It also discusses four categories of challenges to developing an appropriate construction process: (i) lunar simulants; (ii) material fabrication and curing; (iii) microwave-sintering based fabrication; and (iv) fully autonomous and scaled-up construction processes.

Keywords: extra-terrestrial construction process; additive manufacturing; 3D Printing; microwave-sintering; In-Situ Resource Utilisation

1. Introduction The current global interest in Space Odyssey and visiting other planetary bodies in and beyond our solar system has been one of humanity’s oldest dreams from time immemorial. Although John Wilkins (Wilkins 1640) formally documented in 1640 the Moon’s potential as another living environment, the realisation of those dreams only became possible a few decades ago when humans escaped the Earth’s orbit for the first time in 1968 (AIAA 2009). Space-Industry research and enterprise has been subsequently dramatically increased; nowadays, governmental agencies including NASA and ESA and private companies, such as SpaceX and Mars One, plan to send humans to Mars over the next couple of decades and Moon Express will have a rover on the Moon in 2017. As this ambitious plan becomes more detailed and receives greater public exposure through various channels, including the film The Martian, the public, industry, and academic researchers are growing concerned about extra-terrestrial environments and potential technologies providing stable and resilient life-support and shelter for mission crews, tourists, and/or emigres to the Moon and Mars.

3

Such curiosity has led people to consider how best we might provide habitats in unfamiliar and potentially hazardous extra-terrestrial environments, and emergent research and practices in the Space Architecture domain are attempting to eliminate technical barriers for making the dreams come true. Space Architecture can be defined as “the theory and practice of designing and building artificial environments in outer space to enable temporary or permanent human presence or human activities in extreme environments” (Millennium_Charter 2002). It integrates several technical and scientific disciplines with knowledge of space environments, space systems engineering, and the psychology of isolated and confined environments (Osburg et al. 2003). The Space Architecture field was established when architect Maynard Dalton and industrial designer Raymond Lowy persuaded NASA to include a window in their first space station Skylab (AIAA 2009), which was eventually realised when the Skylab was launched in 1973. Other professionals then began to develop the theory and principles of Space Architecture, and it was officially established as a discrete discipline through a peer-reviewed symposium at the 2002 Houston World Space Congress. Space Architecture has thus over the last decade become an emerging topic for future space exploration, and is increasingly seen as a fundamental requirement for supporting long-term (inevitable) space settlement and exploration on other planetary bodies. Those involved in the Space Architecture field believe robotised Additive Manufacturing (AM, a.k.a. 3D Printing) technologies – the process of joining materials to make objects from 3D model data, usually layer upon layer (ASTM 2010) – could become a key technology to construct In-Situ Resource Utilisation (ISRU) human habitation and infrastructure, including radiation protection shielding, surface paving, bridges, dust-shield walls and spacecraft landing pads, etc.

This paper reviews recent and ongoing efforts related to developing ISRU-based construction materials, fabrication methods, and construction processes on other planetary bodies, with particular focus on the lunar surface environment. One of the main reasons for focusing on the development of lunar exploration is its role as a first stepping-stone to setting up human outposts, laboratories, and observatories anywhere between the Kuiper Belt and Mercury (Metzger et al. 2013), and thereafter in deep-space exploration beyond the solar system. NASA already has relatively plentiful geological data and lunar samples compared with other planetary bodies, including Mars, because of the Apollo Missions and the inherent research conducted in the context of the lunar environment.

This paper’s main focus is potential fabrication methods using different techniques (i.e. laser, solar and microwave) for use in AM processes. There are a few more potential areas of extra-terrestrial construction processes, for example, underground construction (for solid surfaced planetary bodies, including the Moon and Mars), underwater construction (for liquid and/or icy surfaced planetary moons, including Europa and Enceladus) construction, assembly of bricks/blocks, and interlocking systems, etc. The technologies used in underground/underwater environments, however, would be significantly different from the solid-surfaced construction reviewed in this manuscript. Therefore, such a comparison between surface, underground and underwater construction for extra-terrestrial environments should be addressed in a separate review paper. In addition, the authors believe that simple shape brick/blocks can be more easily fabricated by casting in a mould, i.e. using a process which is particularly suited for mass production. On the contrary, the strength of AM techniques lies in customisation and complex geometries. Thus, the construction concepts involving the fabrication and assembly of simple-shaped bricks/blocks are deliberately omitted from this review. Furthermore, while the authors are aware of ongoing researches and the practicality of an interlocking system of a space shuttle and extra-terrestrial construction, e.g. (Estrin et al. 2003, Dyskin et al. 2005) which could be broadly included in construction processes, it is part of an assembly process of fabricated components rather than a fabrication method. Thus, techniques of interlocking those bricks/blocks, are also regarded as being outside the scope of this paper.

4

2. Construction materials, fabrication methods and processes It is well-known, that the Moon’s surface environment is extremely hostile for the survival of Earth’s life-forms. Familiar attributes of the lunar surface include: (i) high vulnerability to bombardment by various sizes of micrometeorites and meteorites, due to its virtual high-vacuum environment (Vaniman et al. 1991); (ii) extreme temperature swings – particularly in the equatorial regions – from -178 ˚C to +123 ˚C (Williams et al. 2017); (iii) severe exposure to Solar Proton Events (SPE, a.k.a. extreme solar wind) and Galactic Cosmic Rays (GCR), due to the absence of a magnetic field and atmosphere (Vaniman et al. 1991); and (iv) highly abrasive and electrostatic lunar surface dust produced through space weathering (Metzger et al. 2010). These hostile factors reflect only a fraction of lunar data from multiple lunar missions, and many other as yet unknown factors may present extreme challenges to construction activities on the Moon. Thus, building a human habitat in hostile environments on other planetary bodies will require: (i) the ability and knowledge to locally source and manufacture construction materials, also known as In-Situ Resource Utilisation (ISRU); (ii) fabrication methods appropriate to specific local resources; and (iii) automated construction processes which are optimised for the target environment.

2.1 ISRU based Construction materials The first documented discussion about ISRU took place at the 1981 Case for Mars Conference at the University of Colorado, and concerned producing propellant using local resources to return to the Earth from Mars after an extended exploration mission. NASA later considered building a permanent outpost on the Moon as a base for Manned Mars exploration (NASA 1989), and the concept of ISRU was expanded to the obtaining of resources for building a permanent outpost. Because ISRU is one of the most important concepts in the potential realisation of deep-space exploration and space architecture, a significant amount of ISRU-related research has been carried out over the past four decades (Anand et al. 2012).

Considerable research has been directed at using lunar resources as potential construction materials, as part of an ISRU concept. It is well-known that lunar rocks and ‘soils’ consist of minerals and glasses (Heiken et al. 1991) with a large proportion of silicate minerals. Lunar regolith is a layer of bedrock fragments or debris generated by “the continuous impact of large and small meteorites and the steady bombardment of the lunar surface by charged atomic particles from the sun and the stars” (McKay et al. 1991). Lunar soil is part of lunar regolith, comprised of less than 1 cm in grain size, and it has a well-graded particle-size distribution in geotechnical aspects (Taylor and Meek 2005). Lunar soil is particularly fine grained – over 95 % is categorised as smaller than 1 mm in size, approximately 50 % of which is below 50 µm and 10-20 % below 20 µm (Taylor and Meek 2004), and less than 20 µm size lunar soil is categorised as lunar dust. It is believed that space weathering effectively produces single-domain metallic iron particles (3-33 nm in lunar soil and dust, a.k.a., nanophase Fe0 or np-Fe0) (Keller and McKay 1993, Keller and McKay 1997).

Researchers initially attempted to create conventional construction materials, e.g., cement (Agosto et al. 1988, Yong and Berger 1988, Lin and Bhattacharja 1998), concrete (Lin 1987, Namba et al. 1988, Lin et al. 1997), and brick (Strenski et al. 1990), using resources on the Moon to create a lunar outpost – details of which are summarised in (Ponnada and Singuru 2014, Khitab et al. 2016). However, a conventional concrete production process would require large amounts of water and necessitate a pressurised chamber, as water content tends to evaporate during the “setting” of concrete. Excess evaporation of water can cause formation of severe porosity in the concrete, leading to weakening of the strength and

5

structural failure of the hardened material. Recent works have tried to further extend the potential of lunar concrete by developing a water-less concrete (Toutanji et al. 2005, Koh et al. 2010, Wang et al. 2016), including a sulphur-based concrete (Leonard and Johnson 1988, Omar 1993, Casanova and Gracia 1998, Grugel and Toutanji 2008), phosphoric acid-based concrete (ESA 2017), a new lunar simulant with a special binder to be used with a direct manufacturing technique (Cesaretti et al. 2014), and a glass-fibre based reinforcement for lunar concrete (Tucker et al. 2006). However, ESA’s test of lunar simulant printing with a liquid binder (Cesaretti 2012) indicated the potential freezing of the binder and the related operation with a wet-mix based printing process under the lunar environment’s extreme temperature changes. Despite these valuable efforts, the material treatment and curing process of lunar resources as a construction material still present significant challenges, primarily the use of in-situ materials versus those from Earth. In order to avoid/minimise these challenges, interest has recently grown in exploiting the raw lunar regolith, particularly lunar soil and dust, as a construction material with only minor pre-treatment and liquid-state binder. This would entail using a specific fabrication method: sintering-based additive manufacturing technology.

2.2 Fabrication method – Sintering Sintering is the heating of a porous material to a temperature below its melting point to enable the particles to bond together with a concurrent reduction in the volume of porosity in order to form a solid (Pletka 1993). Some researchers have investigated the potential of lunar regolith sintering using a high-powered laser, solar concentrator, or microwave energies, as the natural lunar regolith is potentially an excellent construction material, consisting mostly of soil and dust (≤1 cm and ≤20 µm, respectively, by lunar conventional definition; (McKay et al. 1991), particles which require only mechanical sieving, without crushing, with minimal disruption of the lunar surface. It was observed that some AM techniques, including Fused Deposition Modelling and Selective Laser (or Solar or Microwave) Sintering, could be used as potential fabrication methods for lunar construction processes (Mueller et al. 2016) and dust mitigation (Wilson and K.B. 2005), because the method does not require any liquid-state binder (Lim and Anand 2015). Some existing research on each method (laser, solar, and microwave) is summarised in the following sections.

2.2.1 Laser and Solar sintering There are relatively few researches on laser sintering of lunar regolith/simulant – only three research works (Balla et al. 2012, Fateri and Gebhardt 2015, Goulas et al. 2017) were found. Despite the thermal transient stresses and residual stresses of direct laser sintering of raw lunar regolith (Balla et al. 2012), the initial experiments with the lunar simulants, e.g. 10 x 25 mm cylinder using JSC-1AC (Balla et al. 2012), 30 x 30 mm net-shape object using JSC-1A (Fateri and Gebhardt 2015), and 20 x 20 x 5 mm cube using JSC-MARS-1A (Goulas et al. 2017), showed that the samples were successfully melted and formed into the desired parts with high geometrical accuracy. Regardless of the interesting results, however, direct laser sintering alone may not be an ideal fabrication method for a construction-scale object for following reasons:

Balla et al. (Balla et al. 2012) claimed that their method requires low energy, 2.12 J/mm2, with a layer thickness of 254 µm, using 50 W of laser power. Note that 2.12J is equivalent to 5.89e-7 kWh, and one meter thickness would need 4,000 layers of 254 µm thickness. Thus, 2.12 J/mm2 would be equivalent to 23,556 kWh/m3 or 12,398 ~ 15,704 kWh/MT (metric tonne), considering the average density of lunar regolith (Heiken et al. 1991) is around 1.5 ~ 1.9 tons/m3 (see Table 1). However, the area and volume of one outpost would easily exceed 260 m2 and 2,000 m3 for

6

ten mission crews (Doule et al. 2011). Thus, the total energy required for construction of the structure protecting the volume of a practical outpost would be extremely high, e.g. requiring a nuclear power source.

Another noticeable drawback of direct laser sintering is that only a small volume of material can be thermally treated at a time, with a longer printing time for a wider area and extremely high-temperature gradients within the sintered component.

However, solar sintering could be a suitable fabrication technique as solar energy is unlimited and readily available on the lunar surface. Researchers investigated the potential of producing lunar glass composite structures (Magoffin and Garvey 1990), lunar concrete (Lin et al. 1997), oxygen production by pyrolysis (Sauerborn et al. 2004), surface stabilisation (Hintze et al. 2009), and lunar brick (Meurisse et al. 2016) using solar energy. Despite the advantages of solar-concentrated sintering methods, it also has its limitations.

The depth penetration of heat from the solar concentrator is better than laser but rather more limited than microwave (up to 13.4 mm for 2.45 GHz microwaves (Allan et al. 2013)). For example, it was reported that solar sintering could penetrate materials up to 6 mm deep in a fixed position or 1-2 mm deep in a raster scanning mode (Hintze et al. 2009, Hintze and Quintana 2013). Thus, it would require much longer times to fabricate than using microwave energy.

The system requires extra complexity, including cleaning mirrors and lenses from lunar dust to sustain the concentrator efficiency (Gaier 2005), and maintaining positioning controls to locate the desired focal spot location relative to the movement of the sun and the solar concentrator (Allan et al. 2013, Hintze and Quintana 2013).

The efficiency of the concentrator may be affected by the optical properties of the lunar regolith. For example, the darker mare regions absorb more light, so it would be heated more efficiently by the solar concentrator than highlands regolith, if all other properties are the same (Hintze and Quintana 2013). Besides, the mare soils melt at considerably lower temperatures than those of the highlands.

Some potential lunar landing-sites, including the Shackleton Crater near the Moon’s south pole may not be exposed to direct sunlight, an environment where a solar concentrator would not be an option.

2.2.2 Microwave sintering Researchers have considered microwave sintering – volumetric heating by absorbing and coupling a microwave field (Agrawal 2006) – because it has significant advantages over conventional heating such as a furnace using fossil fuel and promises substantial energy savings of up to 90% (Sato et al. 2003). Microwave energy can be used for fabricating wider areas, e.g. pavement and/or spacecraft launch and landing pads, etc., and the importance of microwave energy in application to lunar regolith has been made by (Taylor and Meek 2005, Taylor et al. 2010).

Microwave radiation is in the electromagnetic spectrum region with a frequency range between 300 MHz and 300 GHz, and wavelength between 1 mm to 1 m. Researchers tested microwave sintering with lunar regolith or simulant for various purposes, including palaeontological experiments (Hale et al. 1978), volatiles extraction (Etheridge and Kaukler 2011), and nano-phase iron (np-Fe0) production (Tang et al. 2012), etc. For construction purposes Kingery et al (Kingery et al. 1976) believed the complex morphology of the raw lunar regolith might improve sintering because the glass portion of the regolith could assist in densification during sintering, and Meek et al (Meek et al. 1985, Meek et al. 1988) noted

7

that ilmenite (FeTiO3) couples well with high-frequency microwave resulting in densified basalts. Moreover, Pletka (Pletka 1993) observed that sintering the MLS-1 simulant Minnesota Lunar Simulant in a conventional oven is not feasible because the extensive porosity of the sintered specimens led to poor bonding between powdered particles, even though the material was heavily pre-compressed to minimise porosity. He suggested microwave or radiant heating as a viable alternative. Some researchers experimented with the potential of a microwave furnace (Allen et al. 1994) to make lunar bricks. Taylor and Meek (Taylor and Meek 2005) later claimed that the ubiquitous presence of nano-phase iron (np-Fe0) in the raw lunar regolith enhances its suitability for microwave sintering, and hypothesised that the microwave radiation creates ’energy sinks’ with the effective generation of large quantities of heat coupling with np-Fe0 in the lunar soil, using Apollo lunar soil for their experiments. Taylor et al. (Taylor et al. 2010) have an outstanding patent on the microwave heating/sintering/melting of any material containing np-Fe, especially the lunar soil.

Until now, most researches into microwave sintering of both actual lunar soil and lunar simulants have been conducted at 2.45 GHz microwave frequency, which is used for domestic cooking appliances, because of its easily affordable accessibility (Lim and Anand 2015). Actual lunar soil is hard to source because of the limited amount collected from the Moon – only 381.7 kg of rock and soil samples were collected from six US Apollo missions on the near side of the Moon close to the equator – so most researchers have experimented with various lunar simulants. Some thirty simulants had been used for various research purposes at the time of the NASA’s Simulant Working Group survey (LEAG-CAPTEM_SWG 2010), including the commercially produced lunar simulant JSC-1A. Detailed evaluation of existing lunar simulants are also summarised in (Taylor et al. 2016). Simulant JSC-1A was developed with crushed volcanic tuff, including abundant glass and large amounts of nano-meter sized magnetite (Fe2+Fe3+

2O4) grains with similar particle size distribution to that of actual lunar soil (LEAG-CAPTEM_SWG 2010). Although JSC-1A is suitable for certain ISRU activities (Hill et al. 2007), it does not fully represent the geotechnical properties of lunar soil, due to the absence of np-Fe0.

Recent studies involving sintering of lunar simulant also reported that 2.45 GHz microwaves could melt it up to 13.4 mm depth (Allan et al. 2013), and a few researchers (Khoshnevis and Zhang 2012, Barmatz et al. 2014, Lim and Anand 2014, Mueller et al. 2016, Srivastava et al. 2016) have further investigated a microwave sintering technique coupled with a lunar simulant as a potential fabrication method. Considering the composition and dielectric characteristics of lunar regolith, microwave sintering is thought to be one of the most attractive fabrication methods for lunar construction (Srivastava et al. 2016).

2.3 Extra-terrestrial Construction processes using additive manufacturing techniques NASA researchers believe robotics and AM technologies have matured enough for terrestrial applications and have enormous potential to become a game-changer for space colonisation, but an appropriate construction technology will be needed to advance and simplify the complexity of the construction process to fabricate components in order to be adapted in hostile and extreme extra-terrestrial environments. To achieve such advancement and simplicity, they believe AM technologies require advancement beyond their current achievements (Metzger et al. 2013). For example, Kennedy (AIAA 2009) imagined that combining biotechnology with fabric and matrix structures could produce a protective, self-healing and self-regulating system – similar to human skin – optimised for space conditions in the future, which could provide life-support for both people and plants. However, it requires far more advanced technological evolution and a paradigm shifting concept. In order to realise such an ambitious concept, embedding sensors, processors, and mechanical apparatuses in a structure would be a first step toward such intelligent structures; these would autonomously and continuously

8

monitor their own condition and outer environment, including the atmosphere. As AM processes are capable of embedding such functions on the fly, several current research projects in various sectors are developing an advanced concept of extra-terrestrial construction processes beyond current knowledge, which maximises the practicality of the AM processes.

According to (AIAA 2009), extra-terrestrial habitations can be classified in three types (Figure 1): (i) Class I: pre-integrated hard-shell modules such as the International Space Station (Figure 1, left) (AIAA 2009), which are deployed in space with a limited extension capability; (ii) Class II: prefabricated on Earth and surface-assembled modules, e.g. inflatable structures (Figure 1, centre) (NASA 2014); and (iii) Class III: ISRU-derived structures which can be integrated with the other Class I and II modules to expand the habitat (Figure 1, right) (Rousek et al. 2012). The ongoing efforts of several institutions proactively working on these concepts are summarised below.

Figure 1: Three types of extra-terrestrial habitation. Left: Class I (NASA 2001) Image courtesy of NASA;

centre: Class II (NASA 2014) Image courtesy of NASA; right: Class III (Rousek et al. 2012). Note that Rousek (Rousek et al. 2012)’s design is actually a combination of Classes II and III.

2.3.1 USC / NASA JPL – Robotic construction of lunar and Martian infrastructure In early 2000s, the University of Southern California (USC) developed a 3D printer called Contour Crafting (CC) (Khoshnevis 2004), which is capable of fabricating plastic, ceramics, composites, and cementitious objects. This CC utilises an extrusion-based 3D printing technique, similar to the Fused Deposition Modelling (FDM) technique, but using liquid-state binder to crystallise materials instead of heat to sinter materials. Initially, USC exploited the potential of CC with the NASA Jet Propulsion Laboratory (JPL) and the Marshall Space Flight Center (MSFC) for a development of ISRU-based lunar habitat (Khoshnevis et al. 2005), and later further explored the potential of CC embedded on the All-Terrain Hex-Limbed Extra-Terrestrial Explorer (ATHLETE), developed by NASA JPL (Khoshnevis and Zhang 2012) through a NASA Innovative Advanced Concept (NIAC) project. This ATHLETE was developed as a multi-purpose rover, and NASA started to work with USC to develop a combined concept using both CC and ATHLETE for fabricating lunar construction components, such as a protection cover for a lunar habitat, dust-shield walls, and spaceship landing-pads (Howe et al. 2013), through the NIAC initiative. CC and ATHLETE systems have been well demonstrated – their technologies are currently considered to be around Technology Readiness Level (TRL) 3 for space applications – and a preliminary experiment on sulfur concrete using lunar simulant was reported in (Khoshnevis and Zhang 2012). However, more detailed research outcomes have not yet been revealed, possibly because the R&D activities are ongoing.

9

Figure 2: ATHLETE and Contour Crafting: left: Two ATHLETE rovers used for field tests near Moses Lake, WA. (Boston.com 2008), Image courtesy of NASA; right: Construction concept using the CC embedded

on the ATHLETE (Khoshnevis and Zhang 2012)

2.3.2 European Space Agency (ESA) led D-Shape study – 3D printing using a liquid binder In 2010, the European Space Agency (ESA) (Cesaretti 2012, Cesaretti 2012, ESA 2013, Cesaretti et al. 2014) collaborated with the world-renowned UK architecture firm, Foster+Partners (F+P) and D-Shape, to investigate the potential and capability of AM processes to build in-situ structures on the lunar surface using lunar soil as a base material (Figure 3). The F+P firm designed a lunar outpost, and D-Shape provided a gantry-based 6 x 6 metre printing frame to perform the experiment. The D-Shape concept uses a Direct Manufacturing technique, similar to an inkjet printer technique, which produces a three-dimensional object through an additive layering process of materials. The fabrication process consists of: (i) deposition of the sand material for a single layer; (ii) densification of the layered material using a heavy roller; (iii) applying a chlorine-based liquid binder on the layered material following pre-defined printing paths; (iv) curing the bonded layer; and (v) repeating the process until the final layer is reached. A detailed review of the D-Shape technology appears in (Lim et al. 2012). The chosen construction material was a new simulant, DNA-1, manufactured from a quarry in a volcanic structure in Italy, the chemical composition of which is similar to the familiar lunar simulant JSC-1A, but more cost-effective in this instance. The design concept of the lunar outpost, developed by F+P, comprises a simplified modular system (support module + inflatable core module) and a 3D-printed lunar-regolith cover for (micro)meteorites and radiation protection.

10

Figure 3: top: Schematic of the inflatable non-rigid core structure with a support module covered by a

protection cover; below: Section view of the core structure (Cesaretti et al. 2014)

The design iterations and structural feasibility study of the lunar outpost to optimise the concept are described in (Cesaretti et al. 2014). Despite the base architecture’s shortcomings (e.g., deployability and placement of the skylight structures which should not be placed in a high-radiation environment on inhabited system), the ESA team succeeded in designing and testing a closed-cell structure (Figure 4) which both retains loose regolith and ensures shielding from cosmic rays and solar flares.

Figure 4: left: Rendered CAD model of the closed-cell structure (Cesaretti et al. 2014); right: Printed

lunar building-block using a Lunar simulant DNA-1 (Picture reproduced from (ESA 2015)).

2.3.3 European Astronaut Centre (EAC) at ESA – Microwave sintering of lunar simulant Since 2014, extensive experiments on microwave sintering with DNA-1 simulant have been conducted at the European Astronaut Centre (EAC) at ESA. Preliminary results, EAC has suggested that (i) absence of np-Fe0 in lunar simulants does not affect the sintering process, and (ii) additional ilmenite and infrared preheating of the material actually enhanced the reliability and controllability of the sintering process (Häfner 2015).

The main aim of the EAC/ESA experiments was to demonstrate the practicality of the selected 3D printing technologies on the Moon. Through their experiment, EAC/ESA realised that numerous potential issues must be addressed about operation, logistics and economic viability, etc., before planning a first space mission for building the human outpost/habitat on the Moon. Detailed issues are further elaborated in the Discussion section.

11

2.3.4 Made In Space / NASA MSFC – 3D printing in micro- and zero-gravity In 2013 NASA MSFC and Made in Space Inc. demonstrated the potential of 3D Printing in a Zero-G environment using ABS thermoplastic resin materials (Figure 5) over a period of four weeks of microgravity testing with parabolic flights. Although this is not directly related to extra-terrestrial construction processes, the experiments produced some invaluable results to be referred to developing the next generation of AM techniques. The tests enabled measurement of the correlation between gravity and the layer thickness of the printed specimens using commercially available FDM 3D printers: MakerBot and BFB 3000 (Snyder et al. 2013). The initial results show the average layer thickness is (i) around 1.4~1.5 times larger in micro-gravity, (ii) around 1.1~1.15 times larger in lunar gravity (1/6 of the Earth), (iii) slightly larger in Martian gravity (1/3 of the Earth), and (iv) about 0.8 times smaller in 2G compared with the Earth’s gravity. The results indicate that printing on micro-gravity would reduce self-compacting of extruded materials, which possibly leads to weaker bonding (tensile) strengths between layers.

Figure 5: 3D Printing test in zero- and microgravity during a flight aboard a modified Boeing 727 from the

Zero G Corporation. Left: A ‘Made In Space’ experiment box; right: A Microgravity Science Glovebox Engineering Unit. Courtesy of NASA (NASA 2016)

In 2014, NASA launched the 3D printing in Zero-G technology demonstration mission to the International Space Station (ISS, to further explore the potential and capabilities of FDM techniques in micro-gravity environments. NASA used a 3D printer, developed by Made In Space, Inc., to print and compare the same specimen both on the ground and in space using various analyses including photographic/visual inspection, mass and density evaluation, structured light scanning, x-ray and computed tomography, mechanical properties testing, and optical microscopy (Prater et al. 2016). A summary report was published in 2016 with some key lessons learned through the mission, including the need for: (i) a highly controlled fabrication process to isolate the effect of micro-gravity on the FDM process; (ii) a warmed build chamber to minimise distortion of the printed specimens in order to improve the reliability of the FDM process; (iii) a careful strategy on the printing direction depending on the geometry of the components to be printed, in order to improve tensile and flexural strengths of printed specimens; and (iv) a refined microstructural evaluation of printed specimens using a Scanning Electron Microscope (SEM) to optimise the printer settings and fabrication process, e.g. extrusion flow-rate, nozzle correction factor, and curing setting, etc. NASA is also actively promoting a 3D-printing challenge competition as part of NASA’s Centennial Challenges programme (NASA 2015). Through the Phase 1 $50K design competition, NASA selected thirty designs and awarded the top three design finalists. Most designs focus on architectural planning and design with some insights for a construction process. Phase 2 of the competition, which closes in August 2017, is seeking the best technologies utilising indigenous and recyclable materials for the fabrication process.

12

Further to the research outline above, several other activities are directed towards extra-terrestrial construction research. For example, in 2013 The Open University established a new interdisciplinary research initiative called MISSION (http://www.open.ac.uk/researchprojects/mission/), which draws on ideas grounded in the Built Environment discipline. Supported by the concept of 3D Concrete Printing research (Le et al. 2012, Le et al. 2012, Lim et al. 2012, Lim et al. 2016), the research team is focusing on microwave-sintering based additive manufacturing techniques (Lim and Anand 2014, Lim and Anand 2015, Lim et al. 2015, Mueller et al. 2016, Srivastava et al. 2016). In 2015 the German Space Agency (DLR) secured a European Commission supported research project – Regolight (http://regolight.eu) focusing on a solar concentrator based sintering technique (Meurisse et al. 2016).

3. Discussion Despite interesting research developing extra-terrestrial construction processes utilising existing AM technologies, such efforts remain in their infancy, requiring further developments to become a practical application. In order to further development, some challenges particularly relevant to the lunar surface environment and materials are yet to be addressed, some of which are discussed below.

3.1 Lunar simulants Existing researches focus on lunar soil and dust as potential construction materials, which can be collected by scraping up the lunar surface rather than by breaking or crushing rocks. Lunar soil contains various chemical and mineralogical resources – an area of lunar science of considerable on-going importance for ISRU investigations (Anand et al. 2012, Taylor et al. 2016). Due to the lack of availability of actual lunar regolith, developing lunar simulants would be the best way to conduct regolith-related research, – particularly for destructive experiments. In order to support research on raw lunar materials, NASA MSFC (Sibille et al. 2005) identified the criteria for lunar simulant development, which can be used for construction-related experiments on the Earth. There are currently around thirty variations of lunar simulants as mentioned earlier, a few patents relevant to the lunar simulant developments (Hung and McNatt 2010, Weinstein and Wilson 2013), and one ISO standard (ISO 2014). Several simulants – including ALS-1 (Garnock and Bernold 2012), DNA-1 (Cesaretti et al. 2014), and KLS-1 (Ryu et al. 2015), which followed the specification of JSC-1/1A – have been developed for construction purposes. However, experimenting with potential construction materials using existing lunar simulants should not be the only option as they do not represent the full chemical/mineralogical diversity of lunar regolith. For example:

The portion of a lunar core at a depth of 4 cm, acquired by Apollo 16, revealed that lunar regolith can contain large particles (~ 8 mm) (Rickman et al. 2013), and the large particle content of the lunar regolith can reach up to 30% by mass (Iai and Gertsch 2013). However, the existing lunar simulants including JSC-1A only provide a grain size of 1 mm or less, as it is designed mainly for scientific experiments. However, in general, the <1 µm portion of the lunar soil constitutes some 85-95 % of the total soil (McKay et al. 1991).

According to Liu et al. (Liu et al. 2008), lunar regolith has irregular particle shapes, and the specific surface area is approximately 0.5 m2/g, which is about eight times larger than an aggregation of sphere shapes with the same particle size distribution. Park et al. (Park et al. 2008) and Liu et al. (Liu et al. 2008) discussed the particle shape distribution of lunar dust, and concluded that lunar dust contains high percentages of elongated and rugged shapes. Such irregular and rugged shapes of lunar regolith are not considered as one of the criteria for the

13

design of existing simulants, although the shapes could affect the mechanical properties of the fabricated objects using lunar regolith. For example, Barmatz, et al (Barmatz et al. 2013) argued that the sharp-edged particles included in most lunar samples allow more efficient microwave heating than rounder-edged particles mostly found in existing lunar simulants, and speculated that the particle shape may be one of the main reasons for exceptional microwave heating of the lunar regolith as discussed in (Taylor and Meek 2005, Taylor and Liu 2010).

Lunar regolith, particularly lunar dust mainly consists of around 60–80 % of impact-produced glass, i.e. agglutinitic glass (Taylor et al. 2001, Taylor et al. 2005, Taylor and Liu 2010, Taylor et al. 2010), and contains significant quantities of single-domain iron (np-Fe0) (Taylor 1988), which exists as a stable phase (presumably because of the water-poor lunar environment) but is absent in virtually all natural materials on Earth. As a result, the existing lunar simulants, which use natural Earth materials without pre-processing, contain no np-Fe0. Although a few researchers (Liu et al. 2005, Liu et al. 2007, Tang et al. 2010) and a patent (Hung and McNatt 2010) discussed potential methods of synthesising np-Fe0 in a lunar simulant, their ideas have not yet been implemented on a simulant design.

The Apollo landings were restricted to the lunar equatorial regions, so the samples collected represent only limited geographical coverage. The existing simulants have been designed based on the currently available samples. Lunar geology, including the composition of lunar regolith, varies at different locations. For example, the regolith properties of the potential lunar landing site – such as the Shackleton Crater near the Moon’s south pole – would be very different from the Apollo samples used for designing current simulants. This means a single lunar simulant cannot represent the entire lunar surface environment, a conclusion repeatedly stated by (Taylor and Liu 2010, Taylor et al. 2016).

The best way to create an appropriate simulant is by having access to the actual lunar regolith samples from the target-landing site. Several remote-sensing missions have provided global coverage of the lunar surface which provides the best currently available information about the geological/mineralogical characteristics of the target landing-site in the polar regions. However, it is acknowledged that such remote sensing data can only provide information about the very outer lunar surface, and although unlikely, there could be considerable mineralogical/chemical variations with depth. Nevertheless, these are our best approximation until a lander and/or sample return mission to the lunar polar regions takes place. The logical pathway meantime would be to: (i) utilise existing remote sensing data; (ii) source appropriate analogue materials; and (iii) design new simulants or redesign existing simulant compositions for various lunar locations based on the remote sensing data. As was discussed, np-Fe0 may not be the only one element effecting on microwave sintering; thus, a new lunar simulant will need to be designed with/without np-Fe0 and thoroughly tested in both dry and wet conditions to determine whether these elements should be regarded as crucial factors of the new simulant for construction purposes. In reality, it can be other factors in a simulant that give it good microwave coupling – e.g., nanophase magnetite in JSC-1A and other volcanic-based lunar simulants.

3.2 Material fabrication and curing Previous researches at the University of Southern California (Contour Crafting (Khoshnevis 2004, Khoshnevis and Zhang 2012)), Loughborough University (3D Concrete Printing (Lim et al. 2012, Lim et al. 2016)), and the ESA’s recent experiment with D-Shape (Cesaretti et al. 2014) indicate that an extrusion-based fabrication would be the most appropriate method for manufacturing construction components, while Selective Laser Sintering (SLS)-type fabrication would be more suitable for infrastructure development including pavement of roads and launch/landing-pad, etc. The combination of microwave

14

sintering and extrusion-based fabrication seems a logical pathway for this purpose. The challenges raised by existing researches on microwave sintering based fabrication as a practical fabrication method are described below.

Due to the highly abrasive and electrostatic nature of lunar dust and an almost non-existent lunar atmosphere, using lunar soil and dust as a fabrication material would pose significant challenges, particularly on vacuum tribology – friction, lubricant and wear – for a material feeding process prior to sintering and extrusion for fabrication. The overall design of the prototype, including: (i) the material feeding path; (ii) the nozzle size and shape; and (iii) the entire extrusion mechanism, should therefore be optimised to overcome those challenges. A little appreciated factor that will affect microwave applications on the Moon is the 10-12 torr vacuum that will greatly enhance shorting of the microwaves, defusing microwave coupling with the soil components.

Due to weak gravity, less self-compacted materials during deposition will also present a considerable challenge, as they reduce both the density of the fabricated parts and the bonding strengths between layers. It is believed that the compaction of material allows the porosity in the materials to minimise and enables (i) less shrinkage, (ii) a shorter sintering time, and (iii) higher density (Pletka 1993). Thus, the compaction of material prior to fabrication (e.g. sintering and extrusion) is another key criterion for strengths.

As already discussed, there is considerable speculation that the presence of np-Fe0 is one of the main reasons for effective microwave coupling of lunar regolith. As the actual impact of np-Fe0

on microwave sintering remains unidentified, the role of np-Fe0 during microwave sintering process should be carefully investigated.

Once the sintered material is deposited following a pre-defined printing path above the previous layer, the entire layer needs to be hardened enough to support the next layer while preserving a good bonding capability with the next layer. This would be particularly challenging, as there is no conduction/convection on the Moon and heat can only be dissipated through radiation (via infrared waves) in the lunar environment. Clearly, poorly designed curing mechanism for the AM process would affect the printing speed and quality, e.g. deformation of stacked layers. Thus, an efficient curing mechanism – either active or passive – for sintered materials will need to be investigated.

3.3 Microwave-sintering based fabrication This article’s focus is microwave sintering for fabricating construction components in extra-terrestrial environments, particularly on the Moon, as it has unique advantages compared with solar concentration and laser sintering, as already mentioned. However, all the existing works discuss the potential of microwave sintering as an alternative fabrication method without demonstrating its feasibility for use yet as part of AM technologies. Microwave sintering has its own unsolved challenges to becoming an ideal fabrication method. For microwave, higher frequencies penetrate materials less well (i.e. less depth), while lower frequencies penetrate better but are weakly absorbed, so the materials would not absorb enough energy to be adequately sintered, without use of considerably more power. Although 2.45 GHz frequency was specifically chosen for cooking food because of its efficiency in heating liquid water, this frequency may or may not be optimal for sintering other materials, e.g. lunar regolith and simulants. Certain considerations, listed below, need to be addressed in order to maximise the potential of a microwave sintering technique.

15

It is important to understand the factors contributing to the behaviour of lunar soil and simulants when they are heated using microwave energy. Some of these factors relate to the fundamental physical properties of natural materials, which affect and govern the nature of microwave coupling. The absorption of microwave energy depends on the dielectric properties of the material, which are dielectric constant and dielectric loss of lunar soil or lunar simulant. Both properties are major criteria to determine the rate of a material heating in the presence of an electromagnetic wave such as the microwave. However, few research works have investigated the dielectric properties of actual lunar soils from the Chang’e-3 (Feng et al. 2017), the Apollo missions (Olhoeft and Strangway 1975, Strangway and Olhoeft 1977, Olhoeft 1981, Taylor and Meek 2005, Barmatz et al. 2011), and existing lunar simulants (Calla and Rathore 2012, Allan et al. 2013). Existing lunar simulants were not designed to simulate the microwave properties of lunar regolith (Allan et al. 2013). These properties are dependent on the frequency and are also influenced by density, mineral composition, grain size, moisture and temperature, and particle shape, etc. (Calla and Rathore 2012). The dielectric constant of lunar soils is mainly dependent on density at higher frequencies (Olhoeft 1981), and ilmenite contents in the Apollo lunar soils appear to have a correlation with the dielectric constant (Olhoeft and Strangway 1975). On the other hand, the dielectric properties of the lunar simulant JSC-1A were increased with raised temperature at different frequencies (Calla and Rathore 2012). Although these results provide insight into understanding the potential correlation between the density, temperature, and frequencies of microwave sintering, more thorough researches on this subject are needed in order to clarify the appropriateness of the simulant.

It was observed that the microwave radiation using 2.45GHz frequency was not effective for lunar soil and the lunar simulant JSC-1A at lower temperatures between 200 ˚C and 400 ˚C (Allan et al. 2013), then it heated up rapidly at higher temperatures melting the materials completely between 1,100 ˚C and 1,400 ˚C (Allan et al. 2013). In this case, thermal runaway condition (Tiegs et al. 1993), self-insulation (Kenkre et al. 1991), and weak absorption of microwave energy at low temperatures (Allan et al. 2013) would be the major challenges associated with microwave heating to be a realistic fabrication method. Microwave could also be used as a catalyst rather than thermally, and microwave energy is preferentially absorbed by the inner portions, leaving the surfaces less thoroughly heated. Some other form of radiant heating such as the use of a microwave absorbing material (e.g. a susceptor (Allan et al. 2013, Heuguet et al. 2013, Muhammad and Abdullah 2016), which is a material used for absorbing electromagnetic energy and converts it to heat) may need to be combined with the microwave heating to enhance the efficiency of the heating process and temperature uniformity (Allan et al. 2013). With this method, a microwave-susceptor could be used as a catalytic element while laser fuses materials selectively following pre-defined printing paths. Such hybrid heating techniques could also reduce thermal imbalance in the material which can cause weak mechanical properties in the finished product. More research should be carried out to design an appropriate fabrication system.

While addressing the challenges at 2.45 GHz, it is equally important to examine the behaviour of lunar soil at other frequencies in the microwave range. Because of the shallow penetration depth of 2.45 GHz microwave frequency for lunar soil and lunar simulants (Taylor and Meek 2005, Allan et al. 2013) and decreasing depth of penetration when the frequency increases, a set of frequencies might be required for processing different depths of material. In this case, each frequency could be used to change materials in a different phase in order to improve energy efficiency – similar to the concept of Taylor et al (Taylor et al. 2010). Thus, measuring the

16

efficiency of microwave with a different range of frequencies would be the first step to identifying optimal parameters for sintering simulant.

It was observed that the decrease of the grain size causes the increase of the agglutinitic glass content, increasing the presence of np-Fe0 particles in the lunar soil (Taylor et al. 2001, Taylor and Liu 2010, Taylor et al. 2010). This indicates that microwave heating of lunar soil will be improved with smaller grain size due to the abundance of np-Fe0 particles. However, it does not necessarily mean the change in grain size will also affect the microwave heating even in the absence of np-Fe0 particles, which is the case in existing lunar simulants. Varying grain size would also change the dielectric properties of a material, making it an important factor requiring investigation to clarify how it could influence microwave heating of lunar simulants.

In terms of the power requirement, researchers estimated energy consumption for microwave sintering of lunar regolith and simulant, requiring a temperature of up to 1,400 ˚C, around 360 kWh/MT (metric tonne) (Benaroya 2010) or 50,000 kWh/314.159 m2 (20-meter diameter of a landing pad) (Taylor and Meek 2005). Table 1 shows that a laser sintering would require around 33 ~ 44 times more energy than a microwave sintering (compare with Benaroya, Taylor and Meek’s estimates). Note that the given temperature would completely melt most if not all lunar soils and lunar simulants, including JSC-1A. Although the estimate does not reflect the actual energy for microwave power – which appears to be much more energy efficient than laser – it is clear that a powerful energy source is still required, e.g. nuclear energy, considering the construction scale mentioned in Section 2.2.1.

Table 1: Estimated energy consumption of microwave and laser sintering of lunar regolith and simulant

Microwave sintering Laser sintering

Balla et al. (Balla et al. 2012)

N.A.

2.12 J/mm2 with a depth of 254 µm

(Equivalent to 23,556 kWh/m3 or

12,398 ~ 15,704 kWh/MT)

Benaroya (Benaroya 2010)

360 kWh/MT

(Equivalent to 540 ~ 684 kWh/m3) N.A.

Taylor and Meek (Taylor and Meek 2005)

* 50,000 kWh/314.159 m2

(No information regarding the depth) N.A.

Note

MT: Metric tonne

Average density of lunar regolith: 1.5 ~ 1.9 tonnes/m3

* 314.159 m2: 20-meter diameter of a landing pad

3.4 Fully autonomous and scaled-up construction processes Conventional construction processes, which require massive manual intervention, may not be suitable for extra-terrestrial construction due to the extreme and hazardous environments. The initial

17

construction process including an AM-based printing system should therefore be built to be either fully- or semi-autonomous, similar to the present-day operation of lunar and Mars rovers. Unlike those rovers’ current tasks mainly related to material sampling and analysis, however, the proposed printing system needs to: (i) establish infrastructure including stabilising the foundation; (ii) collect construction materials; (iii) sinter the materials for fabrication; and (iv) deposit and cure the sintered materials to build a fairly large habitat module for at least four to ten mission crews. The high complexity of the construction operation would eventually lead to futuristic operations, such as self-sustainable robotic ecologies (Colombano 2004), but developing a fully autonomous and scaled-up printing system for a construction process would be a first step.

Over the last thirty years, improvements in AM materials and processes have resulted in successful commercial realisation. The AM is now an integral part of modern product development (Hague et al. 2003), and the technology has been commercialised to the extent where machines are now affordable for home use. Generally, AM techniques have been applied for relatively small-sized component fabrications. However, several recent research and enterprise efforts have scaled up the AM processes to apply in a construction process despite the slow adoption of new construction technologies and the relatively short history – less than two decades – of AM in construction. Contour Crafting (Khoshnevis et al. 2006) and 3D Concrete Printing (Lim et al. 2012) systems used an extrusion-based AM technique similar to a FDM technique while D-Shape (Dini 2009) used a powder bed (drop-on-powder) technique, and all use a liquid-state binder as a bonding agent. The performance of these systems indicates that a single large-scale printing system may not be an optimal solution for construction processes. Such material fabrication techniques are, however, still only one part of an entire construction process. Other operations, including printing system design strategies, material excavation, collection, delivery, infrastructure, support structure, reinforcement, radiation protection, and location specific design, etc., will also need to be investigated. Some specific concerns should be addressed to realise fully autonomous construction processes:

Some construction activities (including excavation, trenching, backfilling, compacting of materials) are not ideal methods because of lunar dust, which consists of ~20 wt.% of all lunar soils (Taylor et al. 2001, Taylor et al. 2010) and is universally regarded as a design and operational problem (Taylor et al. 2005). Although conventional excavation and drilling activities may not be appropriate in a lunar surface environment, and thus such activities should be minimised, it may not be possible to avoid it completely. A proactive approach to dust mitigation is required, considering the construction scale on the Moon. A few researchers have attempted to mitigate the problem by developing an electrostatic cleaning device (Tatom et al. 1967, Calle et al. 2008, Kawamoto 2012) and a lunar dust collector (Aoyoma and Masuda 1971, Taylor 2002, Afshar-Mohajer et al. 2012), and none are used for real applications yet, particularly for construction processes.

The near-surface lunar environment includes exposure to Solar Proton Events (solar wind) and Galactic Cosmic Radiation (GCR). According to Miller at al. (Miller et al. 2009), who performed the only radiation attenuation measurements using Apollo lunar soils, covering a habitat with a minimum 1.0 m thickness of local regolith, which is somewhat different from NASA’s analysis (AIAA 2009) estimating 2.5 m thickness, would protect mission crews from all SPE and the majority of the most energetic and infrequent highest-energy GCR particles. The shield needs to be solidly fabricated to keep its thickness and shape in order to withstand continuous micrometeoroid impacts. Although the proposed 1.0 and 2.5 m thickness are based on the use of unsintered regolith, which may not be applicable for sintered regolith, it still suggests that a decent thickness of protection cover would be needed. Considering the expected volume of habitat/outpost (Doule et al. 2011), a single printing system may not be an ideal solution to

18

complete an entire protection cover as it would require a long construction duration. Thus, an alternative construction operation scenario, e.g. a collaborative construction using smaller and modular swarm robots, etc., would need to be investigated.

4. Conclusion Space exploration is an expensive undertaking, with return on investment mostly measured in terms of scientific output. However, the global community is increasingly realising the potential of resources available on extra-terrestrial bodies for the future benefit of humankind. Identification of extra-terrestrial resources, and developing techniques to use them, could therefore both reduce our dependence on Earth-based resources and aid in the establishment of financially sustainable space exploration programmes. Moreover, in the longer term, as the Earth’s natural resources continue to be depleted, the emphasis on exploring extra-terrestrial resources will inevitably grow; development of extra-terrestrial construction methods using local planetary resources will be mutually beneficial in both extra-terrestrial and terrestrial settings.

This article has reviewed the existing efforts on developing an extra-terrestrial construction process with three categorisations: (i) ISRU-based construction materials; (ii) fabrication methods; and (iii) construction processes. Current trends in this field focus on sintering-based AM techniques, and microwave is considered the most appropriate alternative method for extra-terrestrial environments, particularly on the Moon. The review of current trends highlights concerns and challenges which need to be addressed in order to develop an appropriate construction process, which are discussed more thoroughly under the following four categories:

Lunar simulants:

The particle size and shape distribution should be matched or at least similar to real lunar regolith.

The need for np-Fe0 in lunar simulants should be carefully investigated, but is probably not required.

Simulant properties should be designed with regards to the regolith of a potential landing site.

Material fabrication and curing:

The printing system should be designed to minimise the vacuum tribology issue with lunar dust, as well as the use of microwave methodology in a high vacuum, such as on the Moon.

Material compaction (or any other similar approach) prior to sintering should be considered to maximise the component strengths.

The actual effect of np-Fe0, particularly for microwave sintering, should be evaluated further.

An effective and efficient curing mechanism needs to be developed.

Microwave-sintering based fabrication:

In-depth studies are needed on correlation between material density, temperature, feedstock composition, and microwave frequencies in microwave sintering.

Some other form of radiant heating, e.g. a susceptor, should be considered to increase microwave sintering efficiency.

The efficiency of microwave with a different range of frequencies should be investigated to identify optimal parameters (e.g., power versus depth) for sintering simulant.

The influence of material grain size on microwave heating should be investigated.

An appropriate energy source for microwave sintering for construction should be identified.

19

Fully autonomous and scaled-up construction processes:

Effective and efficient dust mitigation approaches should be developed.

Fully autonomous and collaborative construction systems and operation strategies should be developed.

Compared with other disciplines, the construction industry is slow to adopt innovation in construction processes and technologies, because it is constantly subjected to the boom-and-bust economic cycle, making it hard for companies to maintain consistent technological progress. A similar situation exists in construction materials, e.g. concrete – first invented by the Ancient Romans (688 BC) and widely used by the Romans from 300 BC to 476 AD – remains one of the main construction materials. The needs of Space Architecture will stimulate the Built Environment industry to address tough construction problems applicable to highly hostile and extreme environments, which may require innovative technologies and materials for an entirely new construction process. With increasing public interest in space travel and settlement supported by academic research and public engagement, the Built Environment and space industries would enjoy new interdisciplinary R&D and enterprise opportunities, leading to new job opportunities in Space Architecture-related businesses. In the Built Environment industry, current growth of high-density populations may eventually force humans to look for alternative habitats on Earth. New human habitation in extreme areas including undersea and Antarctic environments, and improved living standards in the developing world, would thus be viable terrestrial applications of extra-terrestrial construction processes and technologies.

In summary, greater understanding of the unexplored concepts of Space Architecture would enable better use of its value, development, implementation, and evaluation; the success of extra-terrestrial construction processes would eventually support long-term scientific researches on and settlement of other planetary bodies beyond the Moon and Mars. A logical pathway for those purposes is, therefore, to address the important challenges of construction processes in an extra-terrestrial environment, investigate alternative construction processes and materials for Space Architecture particularly on the Moon, and develop an appropriate AM-based printing system.

Acknowledgements This work was supported by The Open University’s Multidisciplinary Research funds for the MISSION (Multidisciplinary Investigation of System using Sintering Instrumentation Of the Next generation) project. Taylor would like to thank the SSERVI Programs of CLASS, at the Univ. of Central Florida, USA, and SEEED, at Brown and MIT Universities, USA.

References Afshar-Mohajer, N., Wu, C.-Y. and Sorloaica-Hickman, N. (2012). "Efficiency determination of an

electrostatic lunar dust collector by discrete element method." Journal of Applied Physics Vol. 112, No. 2. http://dx.doi.org/10.1063/1.4739734.

Agosto, W. N., Wickman, J. H. and James, E. (1988). Lunar Cement/Concrete for Orbital Structures. SPACE 88, Engineering, Construction, and Operations in Space, ASCE, New York, USA.

20

Agrawal, D. (2006). "Microwave sintering of ceremics, composites and metallic materials, and melting of glasses." Transactions of the Indian Ceramic Society Vol. 65, No. 3, pp.129-144. http://dx.doi.org/10.1080/0371750X.2006.11012292.

AIAA (2009). Out of this world: The new field of Space Architecture, American Institute of Aeronautics and Astronautics, Inc. (AIAA).

Allan, S., Braunstein, J., Baranova, I., Vandervoort, N., Fall, M. and Shulman, H. (2013). "Computational Modeling and Experimental Microwave Processing of JSC-1A Lunar Simulant." Journal of Aerospace Engineering Vol. 26, No. 1, pp.143-151. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000245.

Allan, S., Merritt, B., Griffin, B., Hintze, P. and Shulman, H. (2013). "High Temperature Microwave Dielectric Properties and Processing of JSC-1AC Lunar Simulant." Journal of Aerospace Engineering Vol. 26, No. 4, pp.874-881. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000179.

Allen, C. C., Graf, J. C. and McKay, D. S. (1994). Sintering bricks on the Moon. Engineering, Construction, and Operations in Space IV, American Society of Civil Engineers.

Anand, M., Crawford, I. A., Balat-Pichelin, M., Abanades, S., van Westrenen, W., Péraudeau, G., Jaumann, R. and Seboldt, W. (2012). "A brief review of chemical and mineralogical resources on the Moon and likely initial in situ resource utilization (ISRU) applications." Planetary and Space Science Vol. 74, No. 1, pp.42-48. http://dx.doi.org/10.1016/j.pss.2012.08.012.

Aoyoma, M. and Masuda, S. (1971). Characteristics of electric dust collector based on electric curtain. Proceedings of the General Conference of the Institute of Electronic Engineers in Japan.

ASTM (2010). F2792-10 Standard Terminology for Additive Manufacturing Technologies. 100 Barr Harbor Drive, West Conshohocken, PA 19428., American Society for Testing and Materials (ASTM)

Balla, V. K., Roberson, L. B., O'Connor, G. W., Trigwell, S., Bose, S. and Bandyopadhyay, A. (2012). "First demonstration on direct laser fabrication of lunar regolith parts." Rapid Prototyping Journal Vol. 18, No. 6, pp.451-457. http://dx.doi.org/10.1108/13552541211271992.

Barmatz, M., Steinfeld, D., Anderson, M. and Winterhalter, D. (2014). 3D Microwave Print Head Approach for Processing Lunar and Mars Regolith. 45th Lunar and Planetary Science Conference, The Woodlands, Texas, USA, March 17-21.

Barmatz, M., Steinfeld, D., Winterhalter, D., Rickman, D. and Weinstein, M. (2013). Microwave Heating Studies and Instrumentation for Processing Lunar Regolith and Simulants. 44th Lunar and Planetary Science Conference. The Woodlands, Texas, LPI

Barmatz, M. B., Steinfeld, D., Begley, S. B., Winterhalter, D. and Allen, C. (2011). Microwave permittivity and permeability measurements on lunar soils. 42nd Lunar and Planetary Science Conference, Houston, USA, Lunar and Planetary Institute.

Benaroya, H. (2010). Chapter 36: Advanced Systems Concept for Autonomous Construction and Self-Repair of Lunar Surface ISRU Structures. Lunar Settlement. H. Benaroya. Boca Raton, CRC Press.

Boston.com. (2008). "Man on the Moon, Future and Past." The Big Picture, News stories in photographs Retrieved 12th November, 2016, from http://archive.boston.com/bigpicture/2008/07/man_on_the_moon_future_and_pas.html.

Calla, O. P. N. and Rathore, I. S. (2012). "Study of complex dielectric properties of lunar simulants and comparison with Apollo samples at microwave frequencies." Advances in Space Research Vol. 50, No. pp.1607-1614. http://dx.doi.org/10.1016/j.asr.2012.07.031.

Calle, C. I., McFall, J. L., Buhler, C. R., Snyder, S. J., Arens, E. E., Chen, A., Ritz, M. L., Clements, J. S., Fortier, C. R. and Trigwell, S. (2008). Dust Particle Removal by Electrostatic and Dielectrophoretic Forces with Applications to NASA Exploration Missions. Proceedings of ESA Annual Meeting on Electrostatics 2008.

Casanova, I. and Gracia, V. (1998). Sulfur Concrete: A Viable Alternative for Lunar Construction. Space 98. pp.585-591. http://dx.doi.org/10.1061/40339(206)67.

21

Cesaretti, G. (2012). 3D Printed building blocks using Lunar soil - Executive summary (3DP-ALT-ES-0001), European Space Agency

Cesaretti, G. (2012). 3D Printed building blocks using Lunar soil - Final Report (3DP-ALT-RP-0001), European Space Agency

Cesaretti, G., Dini, E., De Kestelier, X., Colla, V. and Pambaguian, L. (2014). "Building components for an outpost on the Lunar soil by means of a novel 3D printing technology." Acta Astronautica Vol. 93, No. 0, pp.430-450. http://dx.doi.org/10.1016/j.actaastro.2013.07.034.

Colombano, S. P. (2004). Self-Sustaining Robotic Ecologies and Space Architecture. Presentation to the AIAA Design Engineering Technical Committee. Reno, Nevada

Dini, E. (2009). "d_shape." Retrieved 6th April, 2009, from http://www.d-shape.com/. Doule, O., Detsis, E. and Ebrahimi, A. (2011). A Lunar base with astronomical observatory (AIAA 2011-

5063). 41st International Conference on Environmental Systems (ICES). Portland, Oregon, USA, AIAA Dyskin, A. V., Estrin, Y., Pasternak, E., Khor, H. C. and Kanel-Belov, A. J. (2005). "The principle of

topological interlocking in extraterrestrial construction." Acta Astronautica Vol. 57, No. 1, pp.10-21. http://dx.doi.org/10.1016/j.actaastro.2004.12.005.

ESA. (2013). "Building a lunar base with 3D Printing." Retrieved 27 June 2013, 2013, from http://www.esa.int/Our_Activities/Technology/Building_a_lunar_base_with_3D_printing.

ESA. (2015). "LUNAR BUILDING BLOCK AT HYPERVITAL." from http://www.esa.int/spaceinimages/Images/2015/04/Lunar_building_block_at_Hypervital.

ESA. (2017). "3D printed Mars simulant." Limited resources manufacturing technologies, Technology Research Programme, from http://www.esa.int/spaceinimages/Images/2017/03/3D_printed_Mars_simulant.

Estrin, Y., Dyskin, A. V., Pasternak, E., Khor, H. C. and Kanel-Belov, A. J. (2003). "Topological interlocking of protective tiles for the space shuttle." Philosophical Magazine Letters Vol. 83, No. 6, pp.351-355. http://dx.doi.org/10.1080/0950083031000120873.

Etheridge, E. C. and Kaukler, W. (2011). Microwave Processing of Planetary Surfaces for the Extraction of Volatiles. 49th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Orlando, Florida, 4-7 January, AIAA. http://dx.doi.org/10.2514/6.2011-612.

Fateri, M. and Gebhardt, A. (2015). "Process Parameters Development of Selective Laser Melting of Lunar Regolith for On-Site Manufacturing Applications." International Journal of Applied Ceramic Technology Vol. 12, No. 1, pp.46-52. http://dx.doi.org/10.1111/ijac.12326.

Feng, J., Su, Y., Ding, C., Xing, S., Dai, S. and Zou, Y. (2017). "Dielectric properties estimation of the lunar regolith at CE-3 landing site using lunar penetrating radar data." Icarus Vol. 284, No. pp.424-430. http://dx.doi.org/10.1016/j.icarus.2016.12.005.

Gaier, J. R. (2005). The effect of lunar dust on EVA systems during the Apollo missions. NASA STI Report Series - NASA Technical Memorandum, TM-2005-213610. Cleveland, NASA

Garnock, B. and Bernold, L. (2012). Experimental Study of Hollow-Core Beams Made with Waterless Concrete. Thirteenth ASCE Aerospace Division Conference on Engineering, Science, Construction, and Operations in Challenging Environments, and the 5th NASA/ASCE Workshop On Granular Materials in Space Exploration, Pasadena, California, United States, 15-18 April. http://dx.doi.org/10.1061/9780784412190.014.

Goulas, A., Binner, J. G. P., Harris, R. A. and Friel, R. J. (2017). "Assessing extraterrestrial regolith material simulants for in-situ resource utilisation based 3D printing." Applied Materials Today Vol. 6, No. pp.54-61. http://dx.doi.org/10.1016/j.apmt.2016.11.004.

Grugel, R. N. and Toutanji, H. (2008). "Sulfur concrete for lunar applications – Sublimation concerns." Advances in Space Research Vol. 41, No. 1, pp.103-112. http://dx.doi.org/10.1016/j.asr.2007.08.018.

Häfner, T. (2015). A Microwave Sinter System for Lunar Regolith Studies - MSc Thesis. MSc, European Astronaut Centre in Cologne, Germany Universit aul Saba er in Toulouse, rance ule University

22

of Technology in Kiruna, Sweden. http://www.diva-portal.org/smash/record.jsf?pid=diva2%3A1032476&dswid=4467.

Hague, R., Campbell, I. and Dickens, P. (2003). "Implications on design of rapid manufacturing." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science Vol. 217, No. 1, pp.25-30. http://dx.doi.org/10.1243/095440603762554587.

Hale, C. J., Fuller, M. and Bailey, R. C. (1978). "On the application of microwave heating to lunar paleointensity determination." Lunar and Planetary Science Conference Proceedings Vol. 9, No. pp.3165-3179.

Heiken, G. H., Vaniman, D. T., French, B. M. and Schmitt, H. H. (1991). Lunar Sourcebook - A user's guide to the Moon. Cambridge, UK, Cambridge University Press.

Heuguet, R., Marinel, S., Thuault, A. and Badev, A. (2013). "Effects of the Susceptor Dielectric Properties on the Microwave Sintering of Alumina." Journal of the American Ceramic Society Vol. 96, No. 12, pp.3728-3736. http://dx.doi.org/10.1111/jace.12623.

Hill, E., Mellin, M. J., Deane, B., Liu, Y. and Taylor, L. A. (2007). "Apollo sample 70051 and high- and low-Ti lunar soil simulants MLS-1A and JSC-1A: Implications for future lunar exploration." Journal of Geophysical Research Vol. 112, No. E02006. http://dx.doi.org/10.1029/2006JE002767.

Hintze, P. E., Curran, J. and Back, T. (2009). Lunar surface stabilization via sintering or the use of heat cured polymers. 47th AIAA Aerospace Science Meeting he New Horizons Forum and Aerospace Exposition. http://dx.doi.org/10.2514/6.2009-1015.

Hintze, P. E. and Quintana, S. (2013). "Building a Lunar or Martian Launch Pad with In Situ Materials: Recent Laboratory and Field Studies." Journal of Aerospace Engineering Vol. 26, No. 1, pp.134-142. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000205.

Howe, A. S., Wilcox, B., McQuin, C., Townsend, J., Rieber, R., Barmatz, M. and Leichty, J. (2013). Faxing Structures to the Moon: Freeform Additive Construction System (FACS) AIAA Space 2013 Conference & Exhibition, San Diego, California, USA, American Institute of Aeronautics and Astronautics. http://dx.doi.org/10.2514/6.2013-5437.

Hung, C.-c. and McNatt, J. (2010) "Lunar dust simulant containing nanophase iron and method for making the same" US 12/711,465, US8283172 B2, http://www.google.com/patents/US8283172.

Iai, M. and Gertsch, L. (2013). "Excavation of Lunar Regolith with Large Grains by Rippers for Improved Excavation Efficiency." Journal of Aerospace Engineering Vol. 26, No. 1, pp.97-104. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000221.

ISO (2014). Space systems - Lunar simulants Kawamoto, H. (2012). "Electrostatic Cleaning Device for Removing Lunar Dust Adhered to Spacesuits."

Journal of Aerospace Engineering Vol. 25, No. 3. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000143.

Keller, L. P. and McKay, D. S. (1993). "Discovery of Vapor Deposits in the Lunar Regolith." Science Vol. 261, No. 5126, pp.1305. http://dx.doi.org/10.1126/science.261.5126.1305.

Keller, L. P. and McKay, D. S. (1997). "The nature and origin of rims on lunar soil grains." Geochimica et Cosmochimica Acta Vol. 61, No. 11, pp.2331-2341. http://dx.doi.org/10.1016/S0016-7037(97)00085-9.

Kenkre, V. M., Skala, L., Weiser, M. W. and Katz, J. D. (1991). "Theory of microwave interactions in ceramic materials: the phenomenon of thermal runaway." Journal of Materials Science Vol. 26, No. 9, pp.2483-2489. http://dx.doi.org/10.1007/BF01130199.

Khitab, A., Anwar, W., Mehmood, I., Kazmi, S. M. S. and Munir, M. J. (2016). "Lunar concrete: Prospects and challenges." Astronomy Reports Vol. 60, No. 2, pp.306-312. http://dx.doi.org/10.1134/S1063772916020050.

23

Khoshnevis, B. (2004). "Automated construction by contour crafting-related robotics and information technologies." Automation in Construction Vol. 13, No. 1, pp.5-19. http://dx.doi.org/10.1016/j.autcon.2003.08.012.

Khoshnevis, B., Bodiford, M. P., Burks, K. H., Ethridge, E., Tucker, D., Kim, W., Toutanji, H. and Fiske, M. R. (2005). Lunar Contour Crafting – A Novel Technique for ISRU- Based Habitat Development. American Institute of Aeronautics and Astronautics Conference. http://dx.doi.org/10.2514/6.2005-538.

Khoshnevis, B., Hwang, D., Yao, K. and Yeh, Z. (2006). "Mega-scale fabrication by contour crafting." International Journal of Industrial and System Engineering Vol. 1, No. 3, pp.301-320. http://dx.doi.org/10.1504/IJISE.2006.009791.

Khoshnevis, B. and Zhang, J. (2012). Extraterrestrial Construction Using Contour Crafting. The 23rd Annual International Solid Freeform (SFF) Symposium, Austin, TX.

Kingery, W. D., Bowen, H. K. and Uhlmann, D. R. (1976). Introduction to ceramics. New York, Wiley. Koh, S. W., Yoo, J., Bernold, L. and Lee, T. S. (2010). Experimental Study of Waterless Concrete for Lunar

Construction. 12th Biennial International Conference on Engineering, Construction, and Operations in Challenging Environments; and Fourth NASA/ARO/ASCE Workshop on Granular Materials in Lunar and Martian Exploration, Honolulu, Hawaii, United States, March 14-17. http://dx.doi.org/10.1061/41096(366)102.

Le, T. T., Austin, S. A., Lim, S., Buswell, R. A., Gibb, A. G. F. and Thorpe, A. (2012). "Mix design and fresh properties for high-performance pritning concrete." RILEM Materials & Structures Vol. 45, No. 8, pp.1221-1232. http://dx.doi.org/10.1617/s11527-012-9828-z.

Le, T. T., Austin, S. A., Lim, S., Buswell, R. A., Law, R., Gibb, A. G. F. and Thorpe, A. (2012). "Hardened properties for high-performance printing concrete." Cement and Concrete Research Journal Vol. 42, No. 3, pp.558-566. http://dx.doi.org/10.1016/j.cemconres.2011.12.003.

LEAG-CAPTEM_SWG (2010). Status of Lunar Regolith Simulants and Demand for Apollo Lunar Samples - Simulant Working Group of the Lunar Exploration Analysis Group and Curation and Analysis Planning Team for Extraterrestrial Materials, NASA

Leonard, R. S. and Johnson, S. W. (1988). Sulfur- Based Construction Materials for Lunar Construction. SPACE 88 Engineering, Construction, and Operations in Space, Proceedings of the ASC, New York, USA.

Lim, S. and Anand, M. (2014). Space Architecture for exploration and settlement on other planetary bodies – In-Situ Resource Utilisation (ISRU) based structures on the Moon. European Lunar Symposium (ELS2014), London, 15-16 May.

Lim, S. and Anand, M. (2015). In-Situ Resource Utilisation (ISRU) derived extra-terrestrial construction processes using sintering-based additive manufacturing techniques – focusing on a lunar surface environment. European Lunar Symposium (ELS2015), Frascati, Italy, 13-14 May.

Lim, S., Anand, M. and Rousek, T. (2015). Estimation of Energy and Material Use of Sintering-Based Construction for a Lunar Outpost — With the Example of SinterHab Module Design. 46th Lunar and Planetary Science Conference (LPSC), Session 817, Woodlands, Texas, 16-20 March.

Lim, S., Buswell, R. A., Le, T. T., Austin, S. A., Gibb, A. G. F. and Thorpe, A. (2012). "Developments in construction-scale additive manufacturing processes " Automation in Construction Vol. 21, No. 1, pp.262-268. http://dx.doi.org/10.1016/j.autcon.2011.06.010.

Lim, S., Buswell, R. A., Valentine, P. J., Piker, D., Austin, S. A. and De Kestelier, X. (2016). "Modelling curved-layered printing paths for fabricating large-scale construction components." Additive Manufacturing Vol. 12, No. B, pp.216-230. http://dx.doi.org/10.1016/j.addma.2016.06.004.

Lin, T. D. (1987). "Concrete for Lunar Base Construction." Concrete International (ACI) Vol. 9, No. 7, pp.48-53.

Lin, T. D. and Bhattacharja, S. (1998). Lunar and Martian Resources Utilization: Cement. Space 98. pp.592-600. http://dx.doi.org/10.1061/40339(206)68.

24

in, T. D., Skaar, S. B. and O’Gallagher, J. J. (1997). " roposed Remote-Control, Solar-Powered Concrete Production Experiment on the Moon." Journal of Aerospace Engineering Vol. 10, No. 2, pp.104–109. http://dx.doi.org/10.1061/(ASCE)0893-1321(1997)10:2(104).

Liu, Y., Park, J., Schnare, D., Hill, E. and Taylor, L. A. (2008). "Characterization of Lunar Dust for Toxicological Studies. II: Texture and Shape Characteristics." Journal of Aerospace Engineering Vol. 21, No. 4, pp.272-279. http://dx.doi.org/10.1061/(ASCE)0893-1321(2008)21:4(272).

Liu, Y., Taylor, L. A., Hill, E. and James, M. D. (2005). Simulation of nanophase iron in lunar soil for use in ISRU studies. 68th Annual Meteoritical Society Meeting.

Liu, Y., Taylor, L. A., Thompson, J. R., Schnare, D. W. and Park, J.-S. (2007). "Unique properties of lunar impact glass: Nanophase metallic Fe synthesis." American Mineralogist Vol. 92, No. 8-9, pp.1420-1427. http://dx.doi.org/10.2138/am.2007.2333.

Magoffin, M. and Garvey, J. (1990). Lunar glass production using concentrated solar energy. Space Programs and Technologies Conference, American Institute of Aeronautics and Astronautics. http://dx.doi.org/10.2514/6.1990-3752.

McKay, D. S., Heiken, G., Basu, A., Blanford, G., Simon, S., Reedy, R., French, B. M. and Papike, J. (1991). The lunar regolith - Chapter 7. Lunar sourcebook - A User's Guild to the Moon. G. H. Heiken, D. T. Vaniman, B. M. French and H. H. Schmitt. Cambridge, UK, Cambridge Univerisity Press. pp.285-356.

Meek, T. T., Fayerweather, L. A., Godbole, M. J., Vaniman, D. T. and Honnell, R. (1988). Sintering lunar simulants using 2.45 GHz radiation. Engineering, Construction, and Operations in Space: Proceedings of Space 1988, New York, American Society of Civil Engineer (ASCE).

Meek, T. T., Vaniman, D. T., Cocks, F. H. and Wright, R. A. (1985). Microwave processing of lunar materials: Potential applications. Lunar Base and Space Activities of the 21st Century. W. W. Mendell. Houston, Lunar and Planetary Institution. pp.479-486.

Metzger, P. T., Lane, J. E., Immer, C. D. and Clements, S. (2010). Cratering and blowing soil by rocket engines during lunar landings. Lunar settlements. H. Benaroya. Boca Raton, FL, CRC Press. pp.551-576. http://dx.doi.org/10.1201/9781420083330-c38.

Metzger, P. T., Muscatello, A., Mueller, R. P. and Mantovani, J. (2013). "Affordable, Rapid Bootstrapping of the Space Industry and Solar System Civilization." Journal of Aerospace Engineering Vol. 26, No. 1, pp.18-29. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000236.

Meurisse, A., Cowley, A., Cristoforetti, S., Makaya, A., Pambaguian, L. and Sperl, M. (2016). Solar 3D Printing of Lunar Regolith. European Lunar Symposium 2016, Amsterdam, the Netherlands.

Millennium_Charter (2002). Fundamental Principles of Space Architecture. Space Architecture Workshop. Houston, TX

Miller, J., Taylor, L., Zeitlin, C., Heilbronn, L., Guetersloh, S., DiGiuseppe, M., Iwata, Y. and Murakami, T. (2009). "Lunar soil as shielding against space radiation." Radiation Measurements Vol. 44, No. 2, pp.163-167. http://dx.doi.org/10.1016/j.radmeas.2009.01.010.

Mueller, R., Howe, S., Kochmann, D., Ali, H., Andersen, C., Burgoyne, H., Chambers, W., Clinton, R., De Kestellier, X., Ebelt, K., Gerner, S., Hofmann, D., Hogstrom, K., Ilves, E., Jerves, A., Keenan, R., Keravala, J., Khoshnevis, B., Lim, S., Metzger, P., Meza, L., Nakamura, T., Nelson, A., Partridge, H., Pettit, D., Pyle, R., Reiners, E., Shapiro, A., Singer, R., Tan, W. L., Vazquez, N., Wilcox, B., Wilkinson, S. and Zelhofer, A. (2016). Automated Additive Construction (AAC) for Earth and Space Using In-situ Resources. Proceedings of the Fifteenth Biennial ASCE Aerospace Division International Conference on Engineering, Science, Construction, and Operations in Challenging Environments (Earth & Space 2016), Orlando, Florida, USA, 11-15 Apr, American Society of Civil Engineers.

Muhammad, W. N. A. B. W. and Abdullah, M. A. B. (2016). "Effect of the Amount and Particle Size of SiC Susceptor on the Properties of Microwave Sintered Magnesium." ARPN Journal of Engineering and Applied Sciences Vol. 11, No. 12, pp.7550-7552.

25

Namba, H., Ishikawa, N., Kanamori, H. and Okada, T. (1988). Concrete Production Method for Construction of Lunar Bases. SPACE 88 Engineering, Construction, and Operations in Space, Proceedings of the ASCE, New York, USA.

NASA (1989). Report of the 90-day study on human exploration of the Moon and Mars NASA (2001). Canadarm2 Maneuvers Quest Airlock, International Space station (ISS), NASA NASA. (2014). "Build a Moon Habitat!" Retrieved 3rd November, 2016, from

http://spaceplace.nasa.gov/moon-habitat/en/. NASA. (2015). "NASA's Centennial Challenges: 3D-Printed Habitat Challenge." Centennial Challenges

Retrieved 11th November, 2016, from http://www.nasa.gov/directorates/spacetech/centennial_challenges/3DPHab/about.html.

NASA. (2016). "3D Printing In Zero-G Technology Demonstration (3D Printing In Zero-G)." Retrieved 3rd November, 2016, from http://www.nasa.gov/mission_pages/station/research/experiments/1115.html.

Olhoeft, G. and Strangway, D. (1975). "Dielectric properties of the first 100 meters of the Moon." Earth and Planetary Science Letters Vol. 24, No. 3, pp.394-404. http://dx.doi.org/10.1016/0012-821X(75)90146-6.

Olhoeft, G. R. (1981). Electrical properties of rocks. Physical properties of rocks and minerals. Y. S. Touloukian, Y. S. Judd and R. F. Roy. pp.257-330.

Omar, H. A. (1993). Production of Lunar Concrete Using Molten Sulfur. Final Research Report for NASA Grant NAG8 - 278, NASA/CR-97-206028, NAS 1.26:206028, NASA

Osburg, J., Adams, C. M. and Sherwood, B. (2003). A Mission Statement for Space Architecture (SAE 2003-01-2431). 33rd International Conference on Environmental Systems (ICES), Vancouver, British Columbia, Canada, 7-10 July 2003. http://dsx.doi.org/10.4271/2003-01-2431.

Park, J., Liu, Y., Kihm, K. and Taylor, L. (2008). "Characterization of Lunar Dust for Toxicological Studies. I: Particle Size Distribution." Journal of Aerospace Engineering Vol. 21, No. 4, pp.266-271. 10.1061/(ASCE)0893-1321(2008)21:4(266).

Pletka, B. J. (1993). Processing of lunar basalt materials. Resources of Near Earth Space, University of Arizona Press. pp.325–350.

Ponnada, M. R. and Singuru, P. (2014). "Advances in manufacture of Mooncrete - A Review." International Journal of Engineering Science & Advanced Technology Vol. 4, No. 5, pp.501-510.

Prater, T. J., Bean, Q. A., Beshears, R. D., Rolin, T. D., Werkheiser, N., Ordonez, E. A., Ryan, R. M. and Ledbetter_III, F. E. (2016). Summary report on phase I results from the 3D Printing in Zero G technology demonstration mission, volume I. NASA Technical Publication

Rickman, D., Edmunson, J. and McLemore, C. (2013). "Functional Comparison of Lunar Regoliths and Their Simulants." Journal of Aerospace Engineering Vol. 26, No. 1, pp.176-182. http://dx.doi.org/10.1061/(ASCE)AS.1943-5525.0000223.

Rousek, T., Eriksson, K. and Doule, O. (2012). "SinterHab." Acta Astronautica Vol. 74, No. 0, pp.98-111. http://dx.doi.org/10.1016/j.actaastro.2011.10.009.

Ryu, B.-H., Baek, Y., Kim, Y.-S. and Chang, I. (2015). "Basic study for a Korean Lunar Simulant (KLS-1) Development." Journal of the Korean Geotechnical Society Vol. 31, No. 7, pp.53-63. http://dx.doi.org/10.7843/kgs.2015.31.7.53.

Sato, M., Muton, T., Shimotuma, T., Ida, K., Motojima, O., Fujiwara, M., Takayama, S., Mizuno, M., Obata, S., Ito, K., Hirai, T. and Shimada, T. (2003). Recent development of microwave kilns for industries in Japan. Proceedings of 3rd World Congress on Microwave and Radio Frequency Applications, Westerville, Ohio, USA, The American Ceramic Society.

Sauerborn, M., Neumann, A., Seboldt, W. and Diekmann, B. (2004). Solar heated vacuum pyrolysis of lunar soil. 35th COSPAR Scientific Assembly, Paris, France, 18-25 July.

26

Sibille, L., Carpenter, P., Schlagheck, R. and French, R. A. (2005). Lunar Regolith Simulant Materials: Recommendations for Standardization, Production and Usage. Huntsville, AL, USA, Marshall Space Flight Center, NASA

Snyder, M., Dunn, J. and Gonzalez, E. (2013). Effects of Microgravity on Extrusion Based Additive Manufacturing. AIAA SPACE 2013 Conference and Exposition, American Institute of Aeronautics and Astronautics. http://dx.doi.org/10.2514/6.2013-5439.

Srivastava, V., Lim, S. and Anand, M. (2016). "Microwave processing of lunar soil for supporting longer-term surface exploration on the Moon." Special Issues, Space Policy Journal Vol. 37, No. 2, pp.92-96. http://dx.doi.org/10.1016/j.spacepol.2016.07.005.

Strangway, D. and Olhoeft, G. (1977). "Electrical properties of planetary surfaces." Philosophical Transactions of the Royal Society of London, Series A: Mathematical, Physical and Engineering Sciences Vol. 285, No. pp.441-450.

Strenski, D., Yankee, S., Holasek, R., Pletka, B. and Hellawell, A. (1990). Brick Design for the Lunar Surface. SPACE 90 Engineering, Construction, and Operations in Space, Proceedings of the ASCE, New York, USA.

Tang, H., Wang, S. and Li, X. (2010). Experimental simulation of nanophase iron production in lunar space weathering. 41st Lunar and Planetary Science Conference

Tang, H., Wang, S. and Li, X. (2012). "Simulation of nanophase iron production in lunar space weathering." Planetary and Space Science Vol. 60, No. 1, pp.322–327. http://dx.doi.org/10.1016/j.pss.2011.10.006.

Tatom, F. B., Srepel, V., Johnson, R. D., Contaxes, N. A., Adams, J. G., Seaman, H. and Cline, B. L. (1967). Lunar Dust Degradation Effects and Removal/Prevention Concepts. NASA Technical Report, TR-792-7- 207A, NASA

Taylor, L. A. (1988). Generation of Native Fe in Lunar Soil. Engineering, Construction, and Operations in Space I, ASCE. New York

Taylor, L. A. (2002) "Method and apparatus for collection of lunar dust particles" 09/ 813630, 20020134399, http://appft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PG01&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&s1=%2220020134399%22.PGNR.&OS=DN/20020134399&RS=DN/20020134399.

Taylor, L. A. and Liu, Y. (2010). Important considerations for lunar soil simulants. Earth and Space 2010: Engineering, Science, Construction, and Operations in Challenging Environments, Honolulu, Hawaii, American Society of Civil Engineers (ASCE). http://dx.doi.org/10.1061/41096(366)14.

Taylor, L. A. and Meek, T. T. (2004). Microwave Processing of Lunar Soil. Proceedings of International Lunar Conference 2003 / International Lunar Exploration Workshop Group 5, American Astronautical Society.

Taylor, L. A. and Meek, T. T. (2005). "Microwave sintering of lunar soil: properties, theory, and practice." Journal of Aerospace Engineering Vol. 18, No. 3, pp.188-196. http://dx.doi.org/10.1061/(ASCE)0893-1321(2005)18:3(188).

Taylor, L. A., Pieters, C., Patchen, A., Taylor, D.-H. S., Morris, R. V., Keller, L. P. and McKay, D. S. (2010). "Mineralogical and chemical characterization of lunar highland soils: Insights into the space weathering of soils on airless bodies." Journal of Geophysical Research: Planets Vol. 115, No. E2, pp.n/a-n/a. http://dx.doi.org/10.1029/2009JE003427.

Taylor, L. A., Pieters, C. M. and Britt, D. (2016). "Evaluations of lunar regolith simulants." Planetary and Space Science Vol. 126, No. pp.1-7. http://dx.doi.org/10.1016/j.pss.2016.04.005.

Taylor, L. A., Pieters, C. M., Keller, L. P., Morris, R. V. and McKay, D. S. (2001). "Lunar Mare Soils: Space weathering and the major effects of surface-correlated nanophase Fe." Journal of Geophysical Research: Planets Vol. 106, No. E11, pp.27985-27999. http://dx.doi.org/10.1029/2000JE001402.

27

Taylor, L. A., Schmitt, H. H., Carrier, W. D. and Nakagawa, M. (2005). The Lunar Dust Problem: From Liability to Asset. First Space Exploration Conference, AIAA 2005–2510. Orlando. http://dx.doi.org/10.2514/6.2005-2510.

Taylor, L. A., Taylor, D. S., Perez, R. F. and DiGiuseppe, M. A. (2010) "Apparatus for In-Situ microwave consolidation of planetary materials containing nano-sized metalic iron particles" US 11/477,253, US7723654 B2, https://www.google.com/patents/US7723654.

Tiegs, T. N., Kiggans, J. O. and Ploetz, K. L. (1993). Application of Microwave Heating for Fabrication of Silicon Nitride Ceramics. Proceedings of the 17th Annual Conference on Composites and Advanced Ceramic Materials: Ceramic Engineering and Science Proceedings, John Wiley & Sons, Inc. pp.744-752. http://dx.doi.org/10.1002/9780470314234.ch7.

Toutanji, H., Glenn-Loper, B. and Schrayshuen, B. I.-M. P. p. (2005). Strength and durability performance of waterless lunar concrete. 43rd AIAA Aerospace Sciences Meeting and Exhibit. http://dx.doi.org/10.2514/6.2005-1436.

Tucker, D., Ethridge, E. and Toutanji, H. (2006). Production of glass fibers for reinforcing lunar concrete. 44th AIAA Aerospace Sciences Meeting. http://dx.doi.org/10.2514/6.2006-523.

Vaniman, D., Reedy, R., Heiken, G., Olhoeft, G. and Mendell, W. (1991). Chapter 3: The Lunar Environment. Lunar Sourcebook - A user's guide to the Moon. G. H. Heiken, D. T. Vaniman, B. M. French and H. H. Schmitt. Cambridge, UK, Cambridge University Press.

Wang, K.-t., Tang, Q., Cui, X.-m., He, Y. and Liu, L.-p. (2016). "Development of near-zero water consumption cement materials via the geopolymerization of tektites and its implication for lunar construction." Scientific Reports Vol. 6, No. pp.29659. http://dx.doi.org/10.1038/srep29659.

Weinstein, M. and Wilson, S. A. (2013) "Apparatus and method for producing a lunar agglutinate simulant " US 12/026,175, US8610024 B1, https://www.google.com/patents/US8610024.

Wilkins, J. (1640). A Discourse Concerning a New World and Another Planet, Iohn Maynard. Williams, J. P., Paige, D. A., Greenhagen, B. T. and Sefton-Nash, E. (2017). "The global surface

temperatures of the Moon as measured by the Diviner Lunar Radiometer Experiment." Icarus Vol. 283, No. pp.300-325. http://dx.doi.org/10.1016/j.icarus.2016.08.012.

Wilson, T. L. and K.B., W. (2005). Regolith Sintering: A Solution to Lunar Dust Mitigation? 36th Annual Lunar and Planetary Science Conference. League City, Texas, USA

Yong, J. F. and Berger, R. L. (1988). Cement-Based Materials for Planetary Materials. SPACE 88, Engineering, Construction, and Operations in Space, ASCE, New York, USA.

28

Highlights

Reviewed current efforts on developing an extra-terrestrial construction process.

The review consists of three categories: materials, fabrication and process.

Discussed the major concerns and challenges for the above efforts.

The above efforts would support permanent settlement of other planetary bodies.

A logical pathway is to develop an appropriate AM-based printing system.